Decoration Increases the Conspicuousness of Raptor Nests

Abstract

Avian nests are frequently concealed or camouflaged, but a number of species builds noticeable nests or use conspicuous materials for nest decoration. In most cases, nest decoration has a role in mate choice or provides thermoregulatory or antiparasitic benefits. In territorial species however, decorations may serve additional or complementary functions, such as extended phenotypic signaling of nest-site occupancy and social status to potential intruders. The latter may benefit both signaler and receiver by minimizing the risk of aggressive interactions, especially in organisms with dangerous weaponry. Support for this hypothesis was recently found in a population of black kites (Milvus migrans), a territorial raptor that decorates its nest with white artificial materials. However, the crucial assumption that nest decorations increased nest-site visibility to conspecifics was not assessed, a key aspect given that black kite nests may be well concealed within the canopy. Here, we used an unmanned aircraft system to take pictures of black kite nests, with and without an experimentally placed decoration, from different altitudes and distances simulating the perspective of a flying and approaching, prospecting intruder. The pictures were shown to human volunteers through a standardized routine to determine whether detection rates varied according the nest decoration status and distance. Decorated nests consistently showed a higher detection frequency and a lower detection-latency, compared to undecorated versions of the same nests. Our results confirm that nest decoration in this species may act as a signaling medium that enhances nest visibility for aerial receivers, even at large distances. This finding complements previous work on this communication system, which showed that nest decoration was a threat informing trespassing conspecifics on the social dominance, territory quality and fighting capabilities of the signaler.

Fig 1. Illustration of the flight plan used during the experiments.

The hexacopter first ascended to 50 m above ground level and took one zenithal image (zenithal snapshot), then flew at 15 m radius 360° around the tree and at 10 m AGL, taking regularly photos (lateral snapshots). Finally, the hexacopter approached the nest making a series of photos (approaching snapshots) at distances of 115, 95, 75, 55, 35, 15 m (30, 26, 22, 18, 14, 10 m AGL, respectively), simulating the aerial perspective of an approaching black kite that intends to inspect the nest.

Fig 2. Images taken by the hexacopter of black kite nests experimentally decorated by the researchers (left side) and of the same nest without decoration (“self-control nest”, right side).

The experimental decoration covered ca 80% of the nest platform, corresponding to a high-level of decoration in real nests, typical of birds in prime age (see [10]). Black arrows indicate the position of the nests. The images have been cropped and enlarged for illustrative purposes.

Fig 3. a) Mean number (± SE) of nests correctly detected by volunteers in standardized surveys of images captured by the hexacopters, and b) mean latency (± SE) to their detection in relation to the decoration treatment (decorated vs non-decorated nests; n = 150 images assessed by 25 volunteers for 15 nests, each measured as decorated or not).

Bars represent 1 SE.

Fig 4. a) Mean number (± SE) of nests correctly detected by volunteers and b) mean latency (± SE) to their detection in relation to the position of the hexacopter (zenithal, lateral and approaching snapshots) when capturing the image and the decoration treatment (decorated vs non-decorated nests; n = 30, 30 and 90 images from zenithal, lateral and approaching snapshots, respectively, assessed by 25 volunteers for 15 nests, each measured as decorated or not).